Remove SyncService and introduce context-based offline mode and state management

- Delete `SyncService` and its associated bidirectional synchronization logic.
- Add multiple context providers (`OfflineProvider`, `AlertProvider`, `LangContext`, `UserContext`, `SessionContext`, `WorldContext`, `SettingBookContext`) for contextual state management.
- Implement `SecureStorage` for OS-level secure data encryption and replace dependency on `SyncService` synchronization.
- Update localization files (`en.json`, `fr.json`) with offline mode and error-related strings.

context/UserContext.ts

@@ -0,0 +1,25 @@
+import {UserProps} from "@/lib/models/User";
+import {Context, createContext} from "react";
+
+export interface UserContextProps {
+    user: UserProps
+}
+
+export const UserContext: Context<UserContextProps> = createContext<UserContextProps>({
+    user: {
+        id: '',
+        name: '',
+        lastName: '',
+        username: '',
+        writingLang: 0,
+        writingLevel: 0,
+        ritePoints: 0,
+	    groupId: 0,
+        aiUsage: 0,
+        apiKeys: {
+            openai: false,
+            anthropic: false,
+            gemini: false,
+        }
+    }
+})